linux-kernel - Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

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Date:	Thu, 13 Aug 2015 19:20:49 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu

13.08.2015 19:09, Andy Lutomirski пишет:
> On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev <stsp@...t.ru> wrote:
>> 13.08.2015 18:38, Andy Lutomirski пишет:
>>>
>>> So...  what do we do about it?  We could revert the whole mess.  We
>>> could tell everyone to fix their DOSEMU, which violates policy and is
>>> especially annoying given how much effort we've put into keeping
>>> 16-bit mode fully functional lately.  We could add yet more heuristics
>>> and teach sigreturn to ignore the saved SS value in sigcontext if the
>>> saved CS is 64-bit and the saved SS is unusable.
>> Andy, why do you constantly ignore the proposal to make
>> new behaviour explicitly controlable? You don't have to agree
>> with it, but you could at least comment on that possibility
>> and/or mention it with the ones you listed above.
> I'm not sure what the proposal is exactly.
>
> We could add a new uc_flags flag.  If set, it means that
> sigcontext->ss is valid and should be used by sigreturn.  If clear,
> then we ignore sigcontext->ss and just restore __USER_DS.
>
> The problem is that, by itself, this won't fix old DOSEMU.  We somehow
> need to either detect that something funny is going on or just leave
> the flag clear by default.
>
> We could do this: always save SS to sigcontext->ss, but only restore
> sigcontext->ss if userspace explicitly sets the flag before sigreturn.
> If we do that, we'd need to also add my patch to preserve the actual
> HW SS selector if possible so that old DOSEMU knows what SS to program
> into its trampoline.
>
> This at least lets *new* DOSEMU set the flag and get the improved
> behavior.  I still don't know what effect it'll have on Wine and CRIU.
>
> Stas, is that what you were thinking, or were you thinking of something else?
Not quite.
I mean the flag that will control not only sigreturn, but
the signal delivery as well. This may probably be a sigaction()
flag or some other. If not set - ss is ignored by both signal
delivery and sigreturn(). If set - ss is saved/restored (and in
the future - also fs/gs).
Is such a flag possible?
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